1. Technical Field
This invention relates to thermopolymeric switching mediums and their use in thermally responsive indicating devices. More particularly, this invention relates to the use of thermopolymeric switching mediums in temperature monitoring devices and to temperature indicating devices that provide a signal upon the attainment of specified temperatures.
2. Prior Art
Thermally responsive indicators are useful in a variety of fields for providing a visual indication of the attainment of a specified temperature. For example, U.S. Pat. No. 4,083,364 discloses a thermal indicator mounted through the skull of an animal to detect the presence of an elevated body temperature. German Patent No. 3229020 discloses a thermally responsive indicator that is designed to be mounted on an electrical conductor to provide a visual indication of the attainment of an elevated temperature in the conductor. U.S. Pat. No. 4,818,119 discloses a railroad wheel bearing bolt with an axially extending bore into which a heat sensor and indicator are inserted in which the indicator is exposed when a specified temperature is attained. U.S. Pat. No. 4,289,088 discloses a sterility indicating device for use in a steam autoclave.
Thermally responsive indicating devices also are useful in preparing food products, particularly meat and fowl. Such devices, such as the Pop Up® brand of disposable temperature indicating devices offered by Volk Enterprises, Inc. of Turlock, Calif., US, can be used to indicate the elevated temperature of the interior of the food product, rather than the temperature of the exterior thereof. By indicating the attainment of a specified internal temperature of the food product, the device can signal when the food product is organoleptically acceptable. These devices must be sufficiently accurate to prevent undercooking or overcooking, which not only may significantly diminish the palatability of the food, but may also even render the food dangerous to eat, as in the case of undercooked meats.
Examples of such indicating devices for use in cooking foods are described in U.S. Pat. Nos. 945978 and 1,509,110, each of which allows a plunger biased by a spring to be released into an extended position upon attaining a specified temperature. Retaining means, which is typically a fusible material, holds the plunger in a retracted position until the fusible material yields, at which time a spring urges the plunger into an extended position. When in the extended position, the plunger provides a visual indication to the user that the food is cooked to an acceptable temperature level and doneness. To further enhance the visibility of the plunger when it is in the extended position, a cap may be attached to the end thereof.
The material of the retaining means has typically comprised metal alloys, as in U.S. Pat. Nos. 3,140,611, 3,682,130, 3,693,579 and 3,713,416 or organic compounds, as in U.S. Pat. Nos. 5,323,730 and 5,537,950. While devices employing such retaining materials have proven useful, they can have certain drawbacks. For example, metal alloys often are more difficult to process.
Organic compounds, such as waxes, have been suggested as replacements for metal alloys. However, waxes typically yield over a relatively wide temperature range, and, if composed of a mixture of different materials, can have multiple melting temperatures, thereby resulting in a premature or a delayed signal. Thus waxes in their current form may not be suitable for temperature indicating devices for use in cooking foods.
A device that attempts to overcome some of the issues exhibited by alloy and wax retaining materials is described in U.S. Pat. No. 4,170,956. This patent discloses a nitrogen-based organic retaining material. The nitrogen-based material used in this device does not exhibit some of the drawbacks of their alloy predecessors and can exhibit better release characteristics than waxes. However, nitrogen-based materials also can have certain drawbacks. For example, because the nitrogen-based materials of U.S. Pat. No. 4,170,956 are very pure when they are prepared, and melting temperature, and therefore yield temperature, tends to increase with increased purity, the materials melt at approximately 84.9° C. when they are tested in their purest state. A turkey is considered done when its interior temperature reaches approximately 82.2° C. Consequently, the nitrogen-based retaining material must be mixed with a foreign substance to reduce its purity. The use of foreign substances adds unnecessary expense. Salts, which are added to the nitrogen-based material to clean it, tend to solidify to form gel globules in the material, thereby requiring that the material be strained. The straining process often must be conducted repeatedly, thereby increasing the time and cost of production. The yield is reduced, and, consequently, a greater amount of material must be manufactured and processed to obtain a given amount of retaining material.
The use of saturated organic compounds is disclosed in U.S. Pat. No. 5,323,730 in which an indicator is disclosed for providing an indication that a food item has attained a desired temperature. The indicator includes an organic retainer comprising a saturated organic compound that maintains a plunger in a retracted position until the material from which the retainer is constructed yields. When the retainer yields, a spring urges the plunger toward an extended position to indicate that the food item has attained the desired temperature.
The use of fatty ketones is disclosed in U.S. Pat. No. 5,537,950 in which an indicator is disclosed for providing an indication that a food item has attained a predetermined temperature. The indicator comprises a barrel having a cavity wherein a plunger is disposed in a retracted position using a retaining material. The retaining material melts when a predetermined temperature is reached releasing the plunger towards an extended position. The retaining material comprises a fatty ketone and at least one other organic compound selected from the group consisting of fatty amides and fatty anilides.
Thermopolymeric switching materials can be used in a variety of commercial products, most notably those that require a well-defined thermal transition from a solid phase to a liquid phase and often thermally cycled multiple times. Natural polymers such as natural rubbers, cellulosic materials and other biologic materials have limited utility for most temperature cycling processes. Synthetic and natural monomeric substances may be obtained that have well defined thermal melting and solidification characteristics, but often are discrete and do not provide a chemistry platform for modification. Synthetic polymers can be produced that can be made to respond to temperature changes under controlled conditions. Synthetic polymeric materials have the advantage of being able to respond within a few degrees to melt or solidify, change their permeability, change their adhesive characteristics, change their biologic properties, as well as number of other property changes often required for product applications.
Moreover, because synthetic polymers can be designed and prepared to function within predetermined ranges of activity using specific modifications within a class of monomeric materials comprising the polymeric composition, synthetic polymers provide a great deal of versatility within a specific class of reagents. More specifically thermopolymeric compositions, with their predictable temperature switching characteristics can find use in temperature monitoring functions where the thermopolymeric material acts as a physical indicator of a temperature event such as heating or cooling.